Alternativa Nº3

According to the Whole Building Design Guide, the steps for designing a BIPV system include consideration of energy-conscious design practices, a choice between a stand-alone and utility-interactive PV system, review of daily electricity use to shift the peak load, providing adequate ventilation of the PVs, consideration of PVs as shading devices following a daylight analysis, designing the system for a local climate and environment (and addressing site planning and orientation issues that may affect this), and reducing the building envelope or other on-site loads (Mike Carter, C.E.T. and Roman Stangl, 2016). For a BIPV/T system, there is also the added consideration of fan use for drawing in air, placement in relation to electrical service areas, and integration with heat recovery systems.

In the bigger scope of designing net zero energy buildings (NZEBs), an integrated approach to all system design is critical. Equally important to designing with renewable energy sources is designing with an energy efficient method. Energy generated by renewables does not necessarily match grid production and can cause load instability when excess electricity is returned (Dermardiros, n.d.). While using batteries for electrical storage is a good option, it is expensive and not always sufficient. Exporting electricity to the grid should be done in conjunction with assessment of peak hours for both energy use and grid demand.

3.6.1 Building Envelope Requirements (Facade Requirements)

Common building typologies and established building envelope techniques guide the BIPV/T design from an architectural standpoint. It is important to consider that as a building material, the PV system must be considered for primary functions of the building envelope including dead, live and other loads, compatibility with other facade materials, thermal insulation performance, air and water resistance, fire safety characteristics, and soundproofing. The insulation of the air cavity of the BIPV/T system should be adjusted to meet thermal and code requirements such as assembly test method NFPA 285 (NFPA, 2012). Due to the size and continuity of the air cavity of this type of system, non-combustible insulation such as stone wool or mineral wool can be used, as well as fire stops at floor edges, to address the concern of vertical and lateral fire propagation on the exterior cladding (Lstiburek, 2017). Also, since the BIPV/T is not producing heat at night, the insulation must be at minimum sufficient as part of a passive wall assembly. This is even more important for systems which rely on the PV glazing thermal

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performance, which is known to have a higher U-value than single glazing (Scognamiglio, 2017).

Other aspects, such as water management, drainage, and insect and bird protection which are especially important to a rainscreen facade, must work in parallel with any electrical/thermal optimizations designed for the air channel.

For the purpose of conventional facade constructability, the installation sequencing and accessibility for maintenance after the system is in place are important. In terms of constraints from the industry, the design is susceptible to PV and PV framing technology limitations, manufacturing availability, and architectural as well as aesthetic aspects (color, texture, finish, visual conformance to other facade appearances) that come to the forefront during design development phases of projects.

3.6.2 PV System – Performance Requirements (Electrical and Thermal Efficiency)

For a BIPV/T system, the electrical efficiency of the system is of primary importance. This is because electrical output produces a higher quality energy than the thermal output. Thermal efficiency is also important and can be enhanced and optimized with methods to increase thermal extraction. Parameters such as building orientation and wall area geometries are less flexible and will affect BIPV/T layout and sizing and system shading, determining the electrical output.

Various PV technologies, such as mono-crystalline silicon or poly-crystalline silicon, will also have an impact on electrical efficiency.

For a given BIPV/T roof length, the temperatures of the PV modules and the air at the outlet will be dependent on solar irradiance, the wind spend and ambient temperature of the environment, and the air velocity inside of the channel (Chen et al., 2010). Cooling of the PV through natural ventilation is typically inefficient as it only occurs through buoyancy or wind, while mechanical cooling with the use of a fan to draw air in must consider the net electrical efficiency after fan energy use is considered. For this, fan optimizations which involve calculating the fan speed and air pressure drop can be employed. Fan consumption is typically not more than 5% of the energy recovered from a BIPV/T system (Athienitis et al., 2010).

Design of the manifold to collect air is an optimization problem of reducing space, pressure drop in the air channel and fan energy consumption. As was shown in the JMSB building, location in relation to the HVAC system is very important. Collected air can be used with heat recovery

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ventilators, water heating in an air-to-water heat exchanger, or directly for space heating.

Depending on proximity to one of these applications, duct sizing and location will be determined.

For electrical efficiency, the effect of PV surface temperatures is reflected in the following equation:

𝜂_𝑒𝑙 = 𝜂_𝑠𝑡𝑐(1 − β_𝑃𝑉(𝑇_{𝑃𝑉,𝑆}− 𝑇_𝑟𝑒𝑓)) (3.1)

where

𝜂_𝑒𝑙 (%), PV electrical efficiency;

𝜂_𝑠𝑡𝑐 (%), PV electrical efficiency under standard testing conditions;

β_𝑃𝑉 (0.45%/C), PV module temperature coefficient for poly-Si (Athienitis et al., 2015);

𝑇_{𝑃𝑉,𝑆} (C), surface temperature of the PV and 𝑇_𝑟𝑒𝑓 (C), PV reference temperature (25C at STC).

This, in turn, affects many aspects of BIPV/T design to focus on reducing the PV temperatures. Even the location of the project in terms of the built environment can have an effect.

In a 2006 study, it was determined that due to urban pollution and consequent reduced radiation on PV surfaces, the PV temperature conversion efficiency is actually improved (Tian et al., 2007).

Besides reducing the PV temperatures, other factors are also important to consider. Frame shadowing, even of a small frame, can significantly decrease the electrical efficiency. In a study on the effects of frame shadows on BIPV/T systems, a 39.3% decrease in electrical efficiency was found as a worst case scenario (Wang et al., 2017).

Figure 3.29 shows a diagram of a thermal network model for a BIPV/T system. The components included in this model are: the PV module, the air channel, the insulation at the back of the channel, and the ambient environment which surrounds the BIPV/T. Energy balance equations determine the interactions between these components to characterize a steady state condition.

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Figure 3.29: Thermal network diagram of a BIPV/T system in section (STPV option is possible) (Yang and Athienitis, 2015)

The total thermal energy extracted by the air flowing through the BIPV/T channel extracts heat from both front and back surfaces:

𝑄_𝑎𝑖𝑟 = 𝑚̇𝑐_𝑝(𝑇_𝑜− 𝑇_𝑖) (3.2)

where

𝑄_𝑎𝑖𝑟 (W), thermal energy recovered by the circulating air;

𝑚̇ (kg/s), mass flow rate of the air;

𝑐_𝑝 (J/kgC), specific heat of the air;

𝑇_𝑜 (C), Temperature of the air at the outlet and 𝑇_𝑖 (C), Temperature of the air at the inlet.

The thermal performance of the system can be enhanced by various techniques, such as double glazing, use of fins, and other methods discussed in the introduction. However, unlike for a solar thermal collector, these optimization and enhancement techniques should be chosen based on the acceptable upper threshold for PV surface temperatures, system durability and taking into account the intended purpose of the system, which may be a heat exchanger, a heat pump, direct integration of the pre-heated air to the air conditioning system in the winter months, or for domestic hot water heating during warm summer months. The selection of the flow rate as well as the heat

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transfer characteristics of the back surface of the air channel can highly impact the performance of the system (Zogou and Stapountzis, 2012).

3.6.3 Standards for BIPV/T Performance

One of the biggest challenges to BIPV/T production is the lack of a unified performance standard. There are currently standards for electric performance of PV panels, air and liquid solar/thermal collector performance, even BIPV performance, but no standard exists for the performance of a PV/T system. Table 3.4 lists some of the existing standards related to PV and BIPV technologies.

Building on some of the existing standards, such as the more recent EN 50583, adding requirements from building envelope testing standards (for air infiltration, water penetration, and thermal performance) and extracting useful correlations from the experimental data, a set of guidelines can be developed for a BIPV/T standard. The system needs to be considered for its passive energy performance as a building envelope component, noting characteristics like the U-value, solar factor and solar heat gain coefficient (SHGC).

It is important to note that the design and optimization of the system’s thermal performance should not be arbitrary and should consider the intended use of the preheated air (heat pump boost, direct preheated fresh air supply, drying, desiccant cooling etc.), which would dictate the requirements for flow rate and outlet air temperature. These requirements, along with the upper limits for maximum PV temperatures, as well as the pumping energy consumption will provide the frame for the performance design of the system.

Table 3.4: Standards related to BIPV/T performance

PV standards

EN 61215 Crystalline Silicon Terrestrial PV Modules

EN 61730 Photovoltaic Module Safety Qualification

UL 1703 Flat-Plate Photovoltaic Modules and Panels

UL 4703 Outline for Photovoltaic Wire

BIPV standards

AC 365 BIPV roof covering systems

EN 50583-1 Photovoltaics in Buildings: BIPV modules

EN 50583-2 Photovoltaics in Buildings: BIPV systems

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Solar thermal standards

ISO 9806 Solar thermal collectors – test methods

Suggestions for BIPV system comprehensive testing include testing for electrical performance, thermal performance and seasonal variation, ventilation performance, visual effect, maintenance criteria, wind resistance, wind-driven rain, and accelerated weathering (R. J. Yang, 2015).

3.6.4 Design concept

Performance enhancements for the BIPV/T cladding, construction limitations and architectural effects were key drivers through the case study design iterations. Figure 3.30 shows a concept render of a multiple-inlet CW system with horizontally oriented PV modules as the outer layer. This concept is the basis for the prototype described in Chapter 4.

Figure 3.30: Multiple-inlet CW BIPV/T system

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Chapter 4